The Processes for producing metal parts from ferrous powders using powder metallurgy (P/M) techniques are well known. Such techniques typically involve mixing of ferrous powders with alloying components such as graphite, copper or nickel in powder form, filling the die with the powder mixture, compacting and shaping of the compact by the application of pressure, and ejecting the compact from the die. The compact is then sintered wherein metallurgical bonds are developed by mass transfer under the influence of heat. The presence of an alloying element enhances the strength and other mechanical properties in the sintered part compared to the ferrous powders alone. When necessary, secondary operations such as sizing, coining, repressing, impregnation, infiltration, machining, joining, etc. are performed on the P/M part.
It is common practice to use a lubricant for the compaction of the ferrous powder. The lubricant can be admixed with the ferrous powders or sprayed onto the die wall before the compaction. The lubricant is used to improve the compressibility of ferrous powders and the uniformity of densification throughout the part. It also reduces the metal powder/die wall friction, and in turn lowers the ejection force that is required to remove the compact from the die, thus minimizing die wear.
Die-wall lubrication is known to lead to compacts with high green strength. Indeed, die-wall lubrication enables mechanical anchoring and metallurgical bonding between particles during compaction. However, die-wall lubrication is not yet widely used because it increases the compaction cycle time, leads to less uniform densification and is not applicable to complex shapes. On the other hand, an admixed lubricant most often reduces the strength of the green compact by forming a lubricant film between the metal particles which limits microwelding and eases the slipping of the particles when stresses are applied.
When complex parts or parts with thin walls are to be produced, as well as when green parts have to be machined, parts with a high green strength are required. A number of patents describe lubricating components leading to compacts with enhanced green strength compared with conventional lubricants such as synthetic waxes and metallic stearates. For example, in U.S. Pat. No. 5,290,336 Luk discloses iron-based powder compositions containing binder-lubricants which increase the strength of green compacts, in terms of transverse rupture strength (TRS) values, up to about 5,000 psi, and which generally reduce the ejection forces during removal of the compacted part from the die cavity. The binder-lubricants comprise a dibasic organic acid and one or more additional components such as solid polyethers, liquid polyethers, and acrylic resins. Such binder-lubricants are added to the iron-based powders preferably in liquid form, dissolved or dispersed in an organic solvent. In U.S. Pat. No. 5,498,276, Luk discloses the use of a polyether or poly(alkylene oxide) in a particulate form as a green strength enhancing lubricant. Green compacts with transverse rupture strength values of about 6,000-7,000 psi are obtained. However, dimensional variations during sintering are higher compared to mixes containing conventional lubricants, which may alter the sintered properties.
Non-sintered soft magnetic parts especially for AC magnetic applications can also be produced using P/M techniques. In this case, the iron-based powder compositions contain an organic dielectric resin which forms an insulating coating between the iron particles and also bind those particles so as to impart mechanical strength to the pressed parts. A wide range of thermoset or thermoplastic resins have been described for the production of such magnetic composites, alone or in conjunction with inorganic insulating coatings, as diclosed for example in U.S. Pat. No. 5,268,140 (Rutz et al.), or European Patent 583,808 (Gay). Different techniques have been used to electrically insulate particles, as disclosed in U.S. Pat. No. 5,211,896 (Ward et al.). Among them, wet techniques, which employ soluble resins, have most often been used to obtain a uniform coating at the surface of the iron particles for high frequency applications. On the other hand, it has been shown that by dry mixing iron and phenolic resin powders and compacting the mix using die wall lubrication, magnetic parts with good permeability and low losses (especially eddy current losses) at frequencies up to 50-100 kHz could be easily obtained. After compaction, compacts are heated at temperatures between 100.degree. C. and 300.degree. C. to crosslink the thermoset resin. The resin has such a low viscosity that it flows inside the compact at the very beginning of the curing treatment to wet, bind and isolate the iron particles. Good mechanical properties, i.e., TRS values as high as 17,000-20,000 psi were obtained.
Even if die wall lubrication can be used to enable the production of soft magnetic iron/resin composites, it is often preferable that the iron-based powder compositions contain an admixed lubricant to improve the processability of such magnetic materials in an industrial environment. Standard known lubricants are, e.g., zinc stearate, amide wax, stearic acid or PTFE, as well as boron nitride for warm compaction at temperatures higher than 250.degree. C. Even if they improve most often the processability of iron/resin powder mixtures, they decrease the strength of pressed parts significantly. There is thus a need for a lubricated powder composition that will give rise to improve processability of soft magnetic composites, while maintaining their good magnetic and mechanical properties.